Centrifugal pumps have flatter head pressure-flow (HQ) curve characteristics when compared to axial flow pumps. Due to these flatter HQ curve characteristics, centrifugal pumps provide a lower head pressure as the flow approaches to zero than axial flow pumps when operated in fixed (constant) speed mode. In certain instances, the aortic pressure may momentarily exceed the pump head pressure and result in a low or even retrograde flow through the pump during diastole. A low centrifugal pump flow in diastole may reduce its ventricular unloading capability.
One possible solutio is employ an axial pump instead of the centrifugal pump in the ventricular assist device. FIG. 1 shows the differences between centrifugal and axial pump HQ curves (head pressure versus flow). There are, however, tradeoffs between axial flow and centrifugal flow pumps. For example, some axial flow pumps are associated with increased likelihood of suction events at lower flow conditions and lower maximum pump flow in systole. Additionally, some clinicians may have a preference for a more sensitive pump with a relatively steeper HQ curve.
Yet another possible solution would be to detect systolic and diastolic phases of the cardiac cycle and adjust pump speed accordingly to provide higher pump pressure during low flow conditions, reducing the likelihood of suction events. But this approach requires special sensors and controllers to very reliably detect the characteristics of the cardiac cycle, and may not even work in low pulsatility conditions which may be present in the case of a severely impaired ventricle. Embodiments of the invention provide solutions which overcome these and other problems.